View full sizeRandy L. Rasmussen/The OregonianA 75-ft section of reinforced steel cage is lowered into place earlier this month as construction continues on the Portland- Milwaukie Light Rail Bridge across the Willamette River. The Ross Island Bridge can be seen in the background.

Opponents of Portland-Milwaukie Light Rail won approval Thursday from Clackamas County election officials to launch a petition drive to require a countywide vote before officials can spend money to finance, design, construct or operate the project.

The county district attorney has five days to review the petition before organizers can begin collecting the 9,378 signatures needed to qualify for the May ballot. The petition calls for public approval for any funding to finance, design, construct or operate the project.

Although the petition probably wouldn't threaten the $1.5 billion line, it could lead to more political and legal wrangling. The project has always faced resistance in the Milwaukie area, where residents have raised concerns over cost, crime, pedestrian safety and changing the character of their neighborhoods.

Even though local, state and federal agencies long ago signed off on the extension and construction began last summer, critics continue to complain that the project never came to a public vote.

"Clackamas County is always in the backseat," said James Knapp, a vocal critic of the light rail project who filed the petition Dec. 21. "Shouldn't we be some kind of force to vote on the big projects?"

Knapp was also part of the group that successfully petitioned to put a $5 Sellwood Bridge vehicle registration fee on the Clackamas County ballot. Voters rejected the fee in May, although it previously had been approved by the county commissioners.

It is unclear what would happen if Knapp and his supporters find similar success with light-rail funding.

In February 2010, Clackamas County officials signed a contract to contribute $25 million to the $1.5 billion project. When critics asked the Clackamas County commissioners in July to refer that commitment to a public vote, county counsel Stephen Madkour warned the commissioners that they risked reneging on a legally binding contract.

If Clackamas County doesn't contribute to the project's cost, TriMet could sue.

"My big concern with this is our commitment, that we have a deal," Clackamas County Commission Chairwoman Charlotte Lehan said in July when residents asked for a vote. "We are committed to the deal because we have so much at stake in terms of our reliability, but I also do believe this is a good project."

Construction began in June on the centerpiece of the project, a 1,720-foot bridge over the Willamette River, and designs are 90 percent complete for the rest of the line. Construction of the east segment could begin as early as February and the 7.3-mile line is slated to open in September 2015.

Knapp, a member of the Oak Lodge Water District board, has been a vocal critic of light rail and an advocate of countywide referendums. He was also bolstered by the success of a citizen-led November ballot measure that requires a countywide vote before creating or making a "substantial change" to urban renewal districts.

Knapp said at least 50 people have volunteered to help gather signatures, including opponents of the Portland-Lake Oswego streetcar and some local business owners, such as Punky Scott of the Bomber Restaurant.

"It's our tax money," Knapp said, "and they won't give us the right to vote on it."